A study has been made on technologies for simulating a wider band signal on a receiver side from a voice signal whose frequency band is narrowed through transmission.
A certain band enhancing technology applies linear prediction analysis to a voice signal to separate the spectrum envelope from sound source, and then generates a high-band signal by transforming the sound source signal through nonlinear processing such as full-wave rectification or half-wave rectification, thereby producing a wider band. Further, the spectrum envelope is converted into a wider-band envelope by using a pre-learned mapping function that maps a narrower-band spectrum envelope to a wider-band spectrum envelope. In this technology known in the art, the wider-band spectrum envelope and the wider-band source signal are combined to generate a wider-band signal.
Further, another technology known in the art applies linear prediction analysis to a voice signal to separate the spectrum envelope from sound source, and obtains a fundamental frequency of the sound source signal to shift the sound source signal to a higher range and to a lower range by a frequency equal to an integer multiple of the fundamental frequency, thereby achieving band broadening.    [Patent Document 1] Japanese Laid-open Patent Publication No. 09-101798    [Patent Document 2] Japanese Laid-open Patent. Publication No. 09-055778